In recent years, cationic electrodeposition paints which are based on epoxy resins and are curable with blocked isocyanates were developed as primers for automotive bodies, etc. Because of their excellent corrosion resistance, they have superseded conventional cationic electrodeposition paints and have been widely used.
Previously, in the coating of an automotive body or the like using such a cationic electrodeposition paint, the three-coat finishing involving cationic electrodeposition primer coating.fwdarw.intermediate coating.fwdarw.top coating has been generally practiced. In recent years, to reduce the coating cost, the decreasing of the number of coating steps has been extensively studied, and it has been strongly desired to develop a coating system by which only a two-coat finishing involving cationic electrodeposition primer coating and top coating can give film properties equivalent to those obtained by the conventional 3-coat finishing. In the two-coat finishing, the corrosion resistance of the coated film can be fully obtained by the electrodeposited coated film. But the weatherability of the coated film is not satisfactory as a result of omitting the intermediate coating. Accordingly, in fields where highly weather-resistant finishes are required as in automobiles, the two-coat finishing cannot be employed.
The present inventors have extensively worked in order to solve the above problem of the two-coat finish by imparting both corrosion resistance and weatherability to the cationic electrodeposition paint used for primer coating. This work has led to the discovery that a cationic electrodeposition bath containing as a binder component a dispersion of a nonionic film-forming resin having excellent weatherability such as an acrylic resin or a polyester resin in an epoxy resin-type cationic electrodepositable resin has excellent storage stability because particles of the nonionic film-forming resin are dispersed very stably by the epoxy-type resin and that by mixing the epoxy-type cationic electrodepositable resin and the nonionic film-forming resin in specific proportions, limiting the surface tensions of these resin components within specific ranges and making the surface tension of the former higher than that of the latter, a cationic electrodeposition bath formed by using the resulting mixture as a binder component gives an electrodeposited film in which upon baking, the nonionic filmforming resin rises to an upper layer portion and the epoxy resin moves toward the surface of the metallic substrate, namely to a lower layer owing to the difference in surface tension; and therefore, as a result, it forms a multilayer film having such a concentration gradient that the upper layer portion is occupied mainly by the nonionic film-forming resin and the lower layer portion, mainly by the epoxy resin. This discovery has led to the present invention.